1. Field of the Invention (Technical Field):
The invention relates to two-phase ternary alloy compositions based on titanium aluminides.
2. Background Art:
Manufacturing industries, particularly the aerospace industry, are in constant need of construction materials that are lightweight, strong, and corrosion and oxidation resistant at high temperatures. It has been recognized in the art that intermetallic aluminides are one of the more promising groups of materials potentially satisfying these requirements while also meeting desirable cost criteria. Aluminum-rich intermetallic alloys, such as titanium trialuminide (TiAl.sub.3), have received substantial attention for their low density and their ability to retain high strength at extreme temperatures. Unfortunately, aluminum-rich intermetallic alloys generally have exhibited brittleness at ambient temperatures. Brittleness in intermetallic aluminides can be attributed primarily to complex and asymmetrical crystal structures; complex crystal structures have an insufficient number of slip systems, which limit bulk deformation behavior. Other reasons (e.g., impeded cross slip, poor intergranular bonding) also contribute to a tendency toward inherent brittleness and low ductility in intermetallic aluminide alloys. Low ductility will render aluminide compositions unacceptable for structural applications.
The intermetallic alloy TiAl.sub.3 is specifically known in the art to have high strength, high hardness, and good heat and oxidation resistance, but also is known to be extremely brittle at ambient temperatures. Some efforts to overcome this shortcoming of an otherwise desirable alloy have been in the area of processing technology. Improved processing methods have not, however, adequately enhanced ductility, a failure most probably attributable to the tetragonal (commonly denoted as DO.sub.22) crystal structure of TiAl.sub.3. Tetragonal crystal structures are among those having less than the requisite number of slip systems necessary for polycrystalline deformation ductility.
It is known that alloys with the symmetric cubic crystal structure (L1.sub.2) possess the required number of slip systems to permit appreciable ductility. It has also been supposed that tetragonal TiAl.sub.3 can be transformed into the more ductile cubic L1.sub.2 crystal structure by the ternary addition of other metallic elements.
U.S. Pat. No. 5,006,054 to Mikkola and U.S. Pat. No. 4,891,184 to Mikkola are related patents disclosing low density, high temperature, aluminum-rich alloys based on modifications of TiAl.sub.3 compositions. The '054 patent teaches the transformation of tetragonal TiAl.sub.3 to the symmetrical cubic L1.sub.2 phase through the addition of manganese and/or chromium to the alloy. The transformation of tetragonal crystal TiAl.sub.3 to cubic crystal L1.sub.2 is presumed and taught. The '054 and '184 patents contain helpful discussion of the state of the art of preparing ternary alloys of aluminum.
U.S. Pat. No. 4,865,666 to Kumar, et al., discloses ternary alloys of TiAl.sub.3 which display the L1.sub.2 cubic structure. Essentially single-phase compositions are disclosed. The '666 patent teaches toward compositions having between eight and fourteen percent atomic weight of Cu, Fe, Co and Ni. The disclosure includes numerous helpful citations to and discussion of past efforts in the art.
U.S. Pat. No. 4,347,076 to Ray, et al., discloses a method of fabricating aluminum alloys using a rapid solidification production processing technique. The rapid solidification process permits the manufacture of very fine powders of aluminum alloys containing various transition metals, which are then consolidated and heat treated to improve their physical properties. No crystal transformation from titanium aluminides is taught, and when Fe is the transition metal of choice, it is included at ten to fifteen weight percent.
U.S. Pat. No. 3,391,999 to Cole, et al., discloses a process for preparing metal aluminides, including titanium aluminide, by reacting the metal with metallic aluminum in a molten salt. The resulting product is brittle, and there is no teaching toward construction uses. There is no disclosure of ternary alloys incorporating Fe or Ni within an L1.sub.2 crystal structure.
U.S. Pat. No. 3,020,154 to Ida discloses a ternary alloy of Al--Ni--Ti, conventionally prepared, where the Ni is between 0.5 and 5.0 weight percent, and the Ti is between 0.5 and 3.5 weight percent.
U.S. Pat. No. 2,919,189 to Nossen, et al., discloses a production process for preparing alloys of refractory metals such as titanium. There is no teaching toward transformation of titanium aluminides to the L1.sub.2 structure by the addition of transition metals.
U.S. Pat. No. 2,750,271 to Cueilleron, et al., discloses a method of preparing Al--Ti alloys; no ternary alloys are taught.
U.S. Pat. No. 2,464,836 to Thomas, et al., discloses an alloy for use in welding rods which may include Al, Fe, Ni and Ti. No structural uses for the alloy are indicated.
German Patent No. 154,485 discloses an Al--Ni--Ti alloy.
Australian Patent No. 221,972 discloses an aluminum alloy containing 0.5 to 2.5 weight percent Ni and 0.1 to 0.3 weight% Ti, the balance being substantially Al. The Australian patent teaches toward the addition of 1.5 weight % or less of Fe or other metals to increase strength.
K. S. Kumar and J. R. Pickens report the preparation of a ternary intermetallic alloy, Al.sub.22 Fe.sub.3 Ti.sub.8, which demonstrated favorable compression strength characteristics. The tensile strength and ductility of the alloy were not disclosed. Kumar, K. S. and Pickens, J. R., "Compression Behavior of the L1.sub.2 Intermetallic Al.sub.22 Fe.sub.3 Ti.sub.8," Scripta Metallurgica, Vol. 22, pp. 1015-1018 (Pergamon Press, 1988).
S. C. Huang, E. L. Hall and M. F. X. Gigliotti discuss a method of rapid solidification processing to prepare TiAl.sub.3 -based alloys containing nickel. S. C. Huang, et al., "Rapidly Solidified Al.sub.3 Ti-base Alloys Containing Ni," Journal of Materials Research, Vol. 3, No. 1, pp. 1-7 (1988).
M. B. Winnicka and R. A. Varin have prepared, and evaluated the compressive ductility of, an intermetallic compound of the Ti--Al--Cu system. M. B. Winnicka and R. A. Varin, "Structure and Compression Behaviour of the L1.sub.2 Al.sub.5 CuTi.sub.2 Intermetallic Compound," Scripta Metallurgica, Vol. 23, pp.1199-1202 (Pergamon Press, 1989).
S. Zhang, J. P. Nic and D. E. Mikkola disclose the formation of cubic crystal alloy phases composed by alloying TiAl.sub.3 with chromium and manganese, with a minimum of second phases. S. Zhang, et al., "New Cubic Phases Formed By Alloying Al.sub.3 Ti With Mn and Cr," Scripta Metallurgica, Vol. 24, pp. 57-62 (Pergamon Press, 1990).
J. P. Nic, S. Zhang and D. E. Mikkola disclose certain structure and property research findings pertaining to certain ternary alloys of TiAl.sub.3 with Cr, Mn, Fe, Co, Ni, Cu and Zn. J. P. Nic, et al., "Observations on the Systematic Alloying of Al.sub.3 Ti With Fourth Period Elements to Yield Cubic Phases," Scripta Metallurgica, Vol. 24, pp. 1099-1104 (Pergamon Press, 1990).
The present invention is better understood with reference to the atomic radius ratio criterion theories expounded by J. H. N. Van Vucht and K. H. J. Buschow. J. H. N. Van Vucht and K. H. J. Buschow, Journal of Less Common Metals, Vol. 10, pp. 98-103 (1965) and Journal of Less Common Metals, Vol. 11, pp. 308-313 (1966).
X-ray diffraction research useful in understanding the results obtained in the examples of the present invention were performed by Hiroshi Mabuchi and colleagues. H. Mabuchi, T. Asai and Y. Nakayama, "Aluminide Coatings on TiAl Compound," Scripta Metallurgica, Vol. 23, pp. 685-689 (Pergamon Press, 1989).
N. Durlu, O. T. Inal (the Applicants), and F. G. Yost include a very brief discussion of the invention in introductory remarks to an article on a related subject. N. Durlu, et al., "L1.sub.2 -Type Ternary Titanium Aluminides of the Composition Ti.sub.25 X.sub.8 Al.sub.67 : TiAl.sub.3 -Based or TiAl.sub.2 -Based?," Scripta Metallurgica, Vol. 25, pp. 2475-2479 (Pergamon Press 1991).
N. Durlu, co-Applicant, teaches many aspects of the present invention in his doctoral dissertation. N. Durlu, Dissertation for the PhD. Degree, "A Study on Aluminum-rich Titanium Aluminides of Iron and Nickel," New Mexico Institute of Mining and Technology Library, which is incorporated herein by reference.
The present invention is disclosed by the Applicants in a recent journal article, including microphotographs of the crystalline structures of the alloys, which is incorporated herein by reference. N. Durlu and O. T. Inal, "Study on TiAl.sub.2 -Based Ternary (Fe or Ni) Titanium Aluminides," Journal of Materials Science, Vol. 27, pp.1175-1178 (Chapman & Hall, 1992).